In most color picture tubes, a peripheral frame, supporting a shadow mask, is suspended in a faceplate panel by means of springs that are welded either directly to the frame or to plates which in turn are welded to the frame. In large size tubes, it is common to use four springs to support a mask-frame assembly within a rectangular faceplate panel of a tube. In many recent tubes, the springs are located at the four corners of the frame to minimize twisting and shifting of the assembly within the panel. An aperture in each spring engages a metal stud that is embedded in an interior corner of a glass panel. An embodiment for achieving such corner support is disclosed in U.S. Pat. No. 4,723,088, issued to Sone et al. on Feb. 2, 1988. That patent shows a mask frame having truncated corners with supports at each corner. In the examples designated "prior art", the mask-frame assembly supports are bent metal plates that are welded to the frame at one end and include an aperture at the other end. The aperture engages a metal stud that is embedded in the panel sidewall. The Sone et al. patent teaches forming each support from two members that are welded together. A first member, having a flat plate shape, is welded at several separated points to a mask frame. The second member is welded to the first member at one end and includes an aperture at the other end that engages a support stud in the panel sidewall.
The use of a corner support system for the support of a color picture tube shadow mask offers many advantages over an on-axis support system. However, the corner support system has the undesirable characteristic of asymmetric resistance to shock loads. Tubes employing corner support systems typically are less capable of sustaining shock loads in the horizontal (X) direction and returning the shadow mask to within a tolerable distance of its original position, than it is for vertical (Y) direction shock loads.
The measure of shock handling capability is a permanent deformation of the mask support system caused by subjecting tube assemblies to X-direction acceleration. Such deformation commonly lead to misalignment of the mask apertures with respect to their nominal positions, which, in turn, causes positional errors in the landings of the electron beams. Such mislandings are commonly referred to as misregistration, and, in operating tubes, the consequences of misregistration are white field nonuniformities and color purity errors. As is well known in the art, the continuous line screen tubes, by their basic operational principal, are very tolerant to Y-direction registration errors, but very intolerant to X-direction registration errors.
As usually mounted on a shadow mask frame, the corner support system springs are typically flexible in the radial direction and very stiff in the tangential direction. These springs are typically mounted at the mask diagonal corners. Because the mask diagonal does not lie at 45 degrees to the X and Y axes, e.g., in a tube having a 4.times.3 or 16.times.9 (X to Y) aspect ratio, the resulting system stiffnesses in the X and Y directions are not equal.
In 4.times.3 aspect ratio tubes, a line connecting the springs on opposite corners makes an angle of 36.87 degrees with respect to the horizontal (X) axis. One skilled in the art would anticipate that the horizontal (X) direction shock handling capability would be about 75% (tan 36.87 degrees) of that in the vertical (Y) direction. For 16.times.9 aspect ratio tubes, the difference between the shock load handling capabilities in the X and Y directions is anticipated to be even greater. The spring diagonal angle for a 16.times.9 aspect ratio tube is usually 29.00 degrees. This indicates that one should expect that the X direction shock handling capability would be about 55% (tan 29.00 degrees) of that in the Y direction. It has been found that the shock handling capability of such a 16.times.9 aspect ratio tube can be unacceptable in many applications. The present invention addresses the problem of inadequate shock load capability in a 16.times.9 aspect ratio tube.